Wearable display systems, and particularly Head Mounted Display (HMD) systems, are well known in the art. HMDs have been conceptualized and manufactured for over 40 years. The first HMD was constructed in the 1960's by Ivan Sutherland. Since Sutherland's 40 pound monochrome HMD, great strides have been made in the field. Most modern augmented-vision HMDs use three general techniques for overlaying information onto an outside real-world environment (augmenting reality or vision). The first technique combines visual data with the outside world utilizing a metalized glass “combiner” optic. This combiner has a very thin layer of metal (usually aluminum), placed over a piece of glass to create a partially slivered mirror. An image is then reflected off this partial mirror and into the users visual field of view. Because of the nature of the mirror, the user is still able to view the outside environment, although somewhat attenuated. This mirror can be curved or flat depending on the particular optical design.
Metalized combiners are among the most robust and versatile combiners. They offer uniform broadband reflectance allowing full color augmented vision. However, they are limited by the very basic physics of reflection and transmission. The portion of transmitted see-through light and the portion of reflected display light must equal 100%. Therefore, if 99% of the light from the outside world reaches the user, only 1% of the total display light will be reflected towards the user. This necessitates the use of a very bright and power hungry display. This general principle makes metalized combiner HMDs impractical for many demanding applications where lightweight, low power devices are required.
Other combining techniques include thin film coatings and Holographic Optical Elements (HOEs). Both of these techniques operate on the principle of reflecting very narrow and specific colors of light to the user's visual field of view. In an optical design these combiners function in a similar manner as metalized combiners. These selective wavelength combiners take a monochromatic light source and direct it to the users visual field of view while letting other colors from the outside environment pass onto the user. If full color augmentation is desired, then three layers of either thin film stacks or three layers of HOEs are required.
Most Military grade HMDs use a thin film coating type of combiner. This type of combiner offers a high degree of see-through vision with a highly reflective monochrome display. The main disadvantage of this type of combiner is that it only works well with narrow band sources, e.g., lasers, or green phosphor CRTs. If full color augmented vision is required, then three narrow band laser sources are needed. These are typically expensive and consume a lot of power. Moreover, even with expensive narrow band reflective coatings, there will still be a noticeable loss of see-through light. There are also other practical considerations such as only having specific angles of incidence over which the coatings will function properly, sometimes as small as +/−5 degrees.
Holographic combiners are plagued by many of the same disadvantages as thin film coatings. Besides being extremely sensitive to color and display light angle of incidents, holographic combiners can also be extremely susceptible to large changes in temperature.
Wire-grid polarizers are also well known in the art. The fist wire-grid polarizer was developed in 1960 by George R. Bird and Maxfield Parrish. However, these polarizers were only for infrared wavelengths. More recently, others have pioneered the use of wire-grid polarizers in the visible spectrum. Wire-grid polarizers are optical elements which work on the principle of transmitting and reflecting linearly polarized light based on its orientation to the wire-grid. Linearly polarized light which is perpendicular to the wire-grid is passed. Linearly polarized light which is parallel to the wire-grid is reflected. These polarizers can reflect over 90% of the polarized light over a large visible spectrum and with angles of incidence greater than +/−20 degrees. One company which presently makes wire grid polarizers for uses in the visible spectrum is Moxtek, Inc., which is also the assignee of U.S. Pat. No. 6,208,463, for a Polarizer Apparatus for Producing a Generally Polarized Beam of Light.
Liquid Crystal on Silicon (LCOS) displays are known in the art, and work by using both pixel by pixel polarization modulation and field sequential color modulation. LCoS displays require linearly polarized light input. Conventionally, wire-grid polarizers have been used with LCoS displays to linearly polarize light before interaction with the LCoS display. It is also known for wire grid polarizers to occasionally be used as a contrast enhancing, or “clean up,” polarizer located after the LCoS display, prior to projecting or viewing the image.